Effect of CO2 concentration on high-temperature carbon fixation and pozzolanic activity of high calcium fly ash

In order to realize the capture of CO2 produced in coal-fired power plants by high-calcium fly ash and to improve the pozzolanic activity of high-calcium fly ash at the same time, the carbon fixation properties of dry carbonation of high-calcium fly ash under different atmospheres were studied in this work. And the effect of carbon fixation on the pozzolanic activity of high-calcium fly ash was revealed. The atmospheres for dry carbonation are air, simulated flue gas, a mixture of O1C4(O2:CO2=1:4), and pure CO2 atmosphere. The physical and chemical structure of high-calcium fly ash before and after carbonation was characterized by using XRD, FTIR, N2 adsorption and SEM-EDS. The carbon fixation properties of high-calcium fly ash were analyzed by using TG. The compressive strength of high-calcium fly ash-cement aggregates after carbon fixation was used as an evaluation index of pozzolanic activity to characterize and analyze the hydration products and microscopic morphology of aggregates at different ages. A preliminary study on the feasibility of high-calcium fly ash as a building material after carbon fixation.The results show that the carbon fixation rates under flue gas, mixed atmosphere, and pure CO2 atmosphere were 7.63%, 8.25%, and 11.03%, respectively. and the contributions of unit CO2 to carbon fixation (the ratio of carbon fixation rate to the CO2 content in the carbonation atmosphere) were 0.44, 0.10, and 0.11, which indicated that the dry carbonation efficiency of high-calcium fly ash is high, and the carbon fixation effect of high-calcium fly ash is optimal under flue gas atmosphere. After carbon fixation in flue gas, mixed atmosphere and pure CO2 atmosphere, The 7 d compressive strength of high-calcium fly ash-cement aggregates increased by 6.4 %, 48.5 % and 72.5 %, respectively, compared with that of the original samples without carbon fixation. And its 28 d compressive strength increased by 7.6 %, 13.8 %, and 44.7 %, respectively. This indicates that carbon fixation has a significant positive effect on the pozzolanic activity of high-calcium fly ash. The reason might be that CaCO3 reacted with hydrated calcium aluminate to generate single-carbon hydrated calcium carbon aluminate. It made that ettringite existed stably and the pore structure was denser to reduce the porosity. So that the compressive strength of high-calcium fly ash aggregates was improved after carbon fixation.